In the oil and gas industry subsea wellbores are drilled from surface vessels, such as drill ships, semi-submersible rigs, jack-up rigs and the like, as is well known in the art. Typically, a drilling riser is provided which extends between the wellhead and a surface vessel to provide a contained passage for equipment and fluids. To this extent the drilling riser normally includes a large bore central riser pipe which accommodates the drilling equipment and certain fluids, such as drilling fluids and wellbore fluids, and a number of auxiliary conduits which extend alongside the central riser pipe and provide communication of control fluids, well kill fluids, choke fluids, hydraulic power fluid and the like. Such auxiliary lines may terminate at the wellhead, for example at a Blow Out Preventer (BOP) or the like.
The drilling riser is typically formed from a number of individual sections or joints which are secured together in end-to-end relation. Each individual section includes the required auxiliary lines arranged around a length of riser pipe, wherein the ends of the riser pipe and auxiliary lines are terminated at opposing flange connectors. During deployment, the individual sections are secured together via the flange connectors. This arrangement permits the riser pipes and auxiliary lines to be connected and sealed together at a single location to speed up the deployment process.
Known drilling risers are of a metallic construction, typically formed from steel. However, it has been proposed in the art, for example from WO 2010/129191 to provide auxiliary lines composed of aluminium.
During use a drilling riser will be subject to various forces. For example, the drilling riser may be subject to bending loads, for example due to deviation of the drilling vessel relative to the wellhead. As the auxiliary lines are offset from the riser bending axis this can result in significant strains being applied within said lines. Further, such bending may result in the auxiliary lines being subject to different levels of strain. For example, an auxiliary line on one side of the riser pipe may be subject to tension during bending of the riser, whereas an auxiliary line on an opposing side may be subject to compression. Excessive bending may result in tensile forces exceeding yield limits, and compressive forces causing buckling within the effected auxiliary line, the result of which may be permanent plastic deformation and/or catastrophic failure. Such deformation or failure may make disassembly difficult, and may prevent subsequent use of the deformed lines. Additionally, these significant differential strains may expose the flange connectors to adverse load conditions.
Furthermore, the drilling riser must be capable of supporting very large tensile forces, primarily applied by its own weight. As the industry moves to deeper waters such global tension requirements are becoming significant. Also, deeper environments place the drilling riser under increasing hoop forces due to large hydrostatic pressures. To accommodate the applied tensile and hoop forces the riser pipe sections must be of very thick wall construction, increasing the weight of the system. System weight will also increase in greater water depths due to the use of longer riser pipe and auxiliary lines. In some situations the design requirements of the riser may result in a system having a weight which exceeds the operational deckload of conventional drilling vessels.
In certain circumstances a rigid connection may be provided between the central riser pipe and the auxiliary lines, such as is disclosed in, for example, U.S. 2001/0017466, U.S. 2011/0073315 and U.S. 2011/0300609. Under static conditions this arrangement might permit acceptable loads to be transferred between the central riser pipe and the auxiliary lines via the rigid connection. However, under dynamic conditions, which is a very important design consideration, it might be possible for the auxiliary lines to become overloaded due to operational forces. For example, different dimensions of the central riser pipe and auxiliary lines may establish a disproportionate effect on the auxiliary lines due to transient loading, such as increasing axial tension and/or compression. Furthermore, the components of known risers are typically formed from metallic components which exhibit relatively large axial stiffness, and as such the reaction of such metallic components to appreciable dynamic loadings might be undesired. For example, the high axial stiffness of such metallic components may result in yield limits being approached or exceeded with relatively low strain levels. That is, an auxiliary line may approach or exceed failure loads during relatively small deformation events. To address such issues it is often the case that safety measures are introduced which permits relative movement between the central riser pipe and auxiliary lines to be achieved, for example during exposure to elevated loads and deformations.
Furthermore, the assembly of known risers having a rigid connection between a central metallic riser pipe and metallic auxiliary lines may be problematic. For example, it is known to fit metallic auxiliary lines between flanges formed integrally at either end of a central metallic riser pipe. However, due to the tolerances in the dimensions of the metallic auxiliary lines and/or the central metallic riser pipe, misalignment between the metallic auxiliary lines and the central metallic riser pipe may occur. This may, for example, necessitate the use of shims, spacers or similar to compensate for mismatches in axial length between the metallic auxiliary lines and the space between the flanges at either end of the central metallic riser pipe. Consequently, the assembly of known risers having a rigid connection between a central metallic riser pipe and metallic auxiliary lines may be complex and time-consuming.